1. Field of the Invention
This invention relates to a transport and/or storage cask for a radioactive material such as spent fuel or the like, and a radiation shield
2. Description of the Related Art
A cask for a radioactive material such as spent fuel from a nuclear power plant or the like must shield gamma rays and neutrons emitted from the radioactive material and to effectively dissipate heat generated through the decay of a radioactive material such as spent fuel or the like contained therein. Examples of such a cask are disclosed, for example, in Japanese Patent Application Laid-Open No. 7-27896 (kokai) and Japanese Patent Application Publication No. 5-39520 kokoku).
A cask disclosed in Japanese Patent Application Laid-Open No. 7-27986 (kokai) has a gamma ray shielding lead layer interposed between an inner shell made of a steel plate and an outer shell made of a steel plate and a neutron shield disposed on the outer surface of the outer shell, so that gamma rays and neutrons emitted form a radioactive material are shielded by these two layers, respectively.
This cask is further provided with cooling fins disposed on the outer side of the neutron shield. The lead layer is closely brought into contact with the outer surface of the inner shell via a thin film of a lead-tin material, thereby efficiently dissipating outward heat generated within the inner shell, such as that resulting from decay of a radioactive material, so that a radioactive material such as spent fuel is transported safely in the cask.
A cask disclosed in Japanese Patent Application Publication No. 5-39520 (kokoku) is also based on the technical thought of shielding gamma rays and neutrons separately, wherein gamma rays emitted from a radioactive material are shielded by carbon steel, and neutrons by a neutron shield.
More specifically, it is composed of a cylindrical vessel made of carbon steel by which gamma rays are shielded, a plurality of metallic heat conductive members which are disposed adjacent to each other around the vessel and between the vessel and the outer shell, and a neutron shield material filing in each of closed spaces formed by the heat-conductive members and the outer shell. Each of the heat conductive members has an L-shaped cross-section and is composed of a portion which extends in the longitudinal direction of the vessel so as to contact the outer surface of the vessel and a portion which extends the radial direction of the vessel and whose end is attached to the inner surface of the outer shell.
The cask disclosed in Japanese Patent Application Laid-Open No. 7-27896 (kokai) has an advantage that the inner shell can be made thin because the lead layer having an excellent shielding capability against gamma rays is disposed between the inner and outer shells, and an advantage that heat generated within the inner shell, such as that resulting from decay of a radioactive material, can be efficiently dissipated outward because the lead layer closely contacts the outer surface of the inner shell via the thin film of a lead-tin material
However, in order to attain a close contact between the lead layer and the outer surface of the inner shell, the lead layer is formed employing a so-called homogeinzing treatment comprising the steps of applying flux containing zinc chloride, stannous chloride, and the like to the outer surface of the inner shell; coating the outer surface with molten lead-tin material; assembling the inner and outer shells together; and casting lead between the inner and outer shells. As a result, the fabrication of the cask takes a longer period of time and involves higher costs.
Further, lead must be carefully cast between the inner and outer shells so as to not introduce defects such as voids, and after casting, the cask must undergo an ultrasonic inspection for such defects.
Moreover, heat generated during casting causes the inner and outer shells to deform, resulting in a nonuniform clearance between the inner and outer shells and thus forming a thinner portion in the thus-cast lead layer. It is therefore necessary to cast more lead than a required quantity corresponding to a required shielding thickness.
The cask disclosed in Japanese Patent Application Publication No. 5-39520 (kokpku) uses a vessel which is made of only carbon steel, thereby shielding gamma rays. When the vessel is made of only carbon steel, the thickness thereof must be considerably large to shield gamma rays because carbon steel is inferior to lead in terms of gamma ray shielding capability. Even though the vessel is relatively thick, the heat-conductive performance thereof is relatively good, and thus no problem arises with respect to heat; however, the vessel's capacity for containing a radioactive material reduces accordingly, resulting in a reduced storage efficiency.
The present inventors proposed, in Japanese Patent Applicaton No. 7-199594, a cask for a radioactive material having a gamma ray shielding layer and a neutron shielding layer disposed on the outer surface of an inner shell, as well as heat conductive members penetrating through the gamma ray shielding layer and neutron shielding layer, as a vessel having a high efficiency of storing a radioactive material and an excellent heat-conductive performance.
This transport/storage cask for a radioactive material, a transverse cross-secton of which is typically shown in FIG. 8, is composed of a vessel inner shell 1 having a basket 5 for containing a radioactive material, an outer shell 2, and a gamma ray shield layer 3 and a neutron shield layer 4 successively disposed between the vessel inner shell 1 and the outer shell 2.
According to this invention, the heat-conductive performance of the vessel is particularly excellent, compared with the above-mentioned prior arts, by the presence of the heat conductive members.
When it is required to shield both gamma rays and neutrons, as in a cask for spent fuel radiation source intensities of gamma rays and neutrons are differed depending on spent fuel, and it is important to effectively shield gamma rays and neutrons according to the balance of radiation source intensity between the both or the degree of intensity.
In the above-mentioned structures of the prior arts, since gamma rays and neutrons are shielded independently by a gamma ray shielding layer and a neutron shielding layer, respectively, it is difficult to design the material or thickness of each shielding layer according to the balance of radiation source intensity. Therefore, a conservative design such as thickening of the respective shielding layers is needed, resulting in an increase in size of the vessel, and construction of the shielding layers and the heat conductive members is obliged to be complicated.